If you no longer require a folder & need to delete the folders from your Yahoo account. Then you can easily delete it in Yahoo Mail.
To remove a personalized file from Yahoo mail:
1. Open the folder you want to delete.
2. Transfer messages to other files or delete them until the file is empty.
You cannot delete a file that still has mail in it.
Maybe Yahoo Postal account via IMAP in an email program to quickly transfer or archive messages.
You can also delete files using Yahoo IMAP mail into any email program, of course, and remove them from Yahoo Mail online as well as other account-related programs using IMAP.
This enables you to delete files including their contents; note, however, that deleted messages may not appear on Yahoo! Internet Trash Folder - though your email program may have moved them to a deleted local folder.
See here (under Yahoo Mail Basic) for another way to delete a file and all its messages in one step.
3. Click on the folder in the list of files with the right mouse button.
4. Select Delete from the menu.
If you unintentionally delete an empty folder:
Click undo soon after it appears at the top of the Yahoo Mail screen.
To delete a custom folder from Yahoo Postal account using Yahoo Mail Basic:
1. Start the file you want to delete on Yahoo Basic Mail.
2. Search and scroll any message you want to keep.
3. Click [Edit] next to my files in the listing of files.
4. Click Delete next to the folder you want to remove under My Folders.
Note that, in Yahoo Mail Basic, you do not need to empty the file before deleting it; any message left in the folder will be transferred to the Trash folder, from which you can recover them, of course, if someone has been unintentionally removed.
5. Click OK under Delete Folder.
Read More:- How to Create Folder in Yahoo Mail